Cyclotron target



April 18, 1950 A. F. REID CYCLOTRON TARGET Filed Jan. 26, 1945 WATERINLET WATER OUTLET Fig.2.

WATER INLET INVENTOR. 'ALLEN F. REID cyclotron.

?atented Apr. 18, 1950 STAT S PA? CYCLOTRON TARGET Application January26, 1945, Serial No. 574,807

7 Claims.

1 This invention relates to the production of radioactive materials bybombardment with high- ,velocity subatomic particles and moreparinterest in the production of radioactive isotopes of variouselements by bombardment of a suitable target placed in a stream ofhigh-velocity subatomic particles produced, for example, by aRadioelements may be prepared by bombardment with particles such asdeuterons or protons of a target composed of or comprising an elementsuitably selected to cause a nuclear reaction producing the desiredradioactive iso tone. The target element may be the same element as theradioisotope produced or may be a different element. In cases where thebombarding particles are produced by a cyclotron, the target may beinserted into the cyclotron casing where the intensity and energy of thebeam of particles are relatively high or the beam may be brought out ofthe cyclotron through a suitable window 01' opening and caused toimpinge on the target at a point outside the cyclotron casing. In mostcases greater efiiciency can be -,obtained by inserting the target intothe cyclotron and in some cases the desired nuclear reaction will notproceed at all or will proceed only to a negligible extent unless thetarget is :50 inserted.

Due to the severe conditions existing within the cyclotron duringbombardment, i. e. the

high energy level and intensity of the beam, the preparation of suitabletargets has presented a serious problem, particularly in those caseswhere unusually high energies are required to cause the nuclear reactionproducing the desired radioelement to proceed, and bombardment withinthe cyclotron is mandatory if reasonably efficient conversions are to beobtained.

The character of the beam is such that it tends to heat the target tovery high temperatures which may fuse or vaporize one or more componentsof the target and the physical impact of the bombarding particles tendsto cause mechanical disintegration of the target. In order to obtaindesirable yields of radioactive materials in such cases, the targetshould be suificiently refractory to withstand the high temperaturesgenerated by the incident beam without melting,

vaporizing, or decomposing chemically and should 2 be sufiicientlycoherent to withstand the impact of the bombarding particles withoutmechanical disintegration.

For the purpose of reducing undesired temperature efiects it has beenthe common practice to prepare targets comprising a relatively thinlayer of the material to be bombarded on a heatconductive base, and tocool the base and associated layer with a rapidly flowing stream orcooling water. However the energy of the incident beam is so great thateven when an attempt is made to cool the target in this manner, thetarget temperatures rises to values sufiicient to vaporize many elementsand compounds.

It is apparent that mechanical disintegration of the target should beavoided if high yields of the radioelement are to be obtained. When suchdisintegration occurs the target material may be scattered by impact ofthe incident particles. Furthermore, since atoms of the radioactivematerial are formed by collision of the bombarding particles, with atomsof the target element, it is evident that when the target material lackscoherence the probability that an atom of the radioactive material willbe scattered is relatively greater than the probability that an atom ofthe target element will be scattered.

One of the radioisotopes that is of considerable interest is radioiodine(I which has a half life of 8 days. This isotope of iodine may be usedfor a variety of purposes including, for example, its use as a tracer inpathological studies of the utilization of iodine by the human body. Inpreparing radioiodine it has been customary to bombard a targetcomprising an alloy of copper, cobalt and tellurium containing somewhatmore than 50% tellurium with a stream of high-Velocity deuterons in acyclotron. Several nuclear reactions occur which may be represented bythe following equations, the first reaction being the predominant one.

A relatively high energy beam is required to cause these reactions toproceed satisfactorily and therefore the reactions are desirably carriedout within the cyclotron.

The conventional copper-cobalt-tellurium target has the advantage thatit contains a relatively large proportion of tellurium. However itsphysical character is such that a considerable portion of the activityin the target is either vaporized or otherwise dissipated duringexposure It is still another object of "the invention to i provide amethod of making such a target.

It is a still further object .of the invention to provide a method ofproducing radioiodine and/ or other radioelements by bombardment of atarget comprising thallous tellurite.

Other objects of the invention Will be in par-t obvious and in partpointed out hereinafter.

In one of its broader aspects the present invention comprises animproved target for use in the production of radioisotopes of iodineand/or other elements by bombardment with high-ve- *loclty sub-atomicparticles-such as deuterons, the target comprising a heat-conductivebasehaving a-relativel-y thin, adherent layer of thallium tellurite thereon.It has been found that thallous tellurite when properly applied to asuitable supporting base is unusually resistant to the conditionsexisting within a cyclotron during bombardment and that a relativelyhigh yield of :radioiodine may be obtained when this material is usedasa target.

The thallous tellurite is preferably used in the tormof a relativelythin, vitreous layer on aheatconducting base such as -=copper. Theoptimum thickness of the layer is largely determined by thecharacteristics of the bombarding particles and the layer should be ofsuch a thickness as to utilize these particles with maximum effective-:ness. As the bombarding particles penetrate the target layer theirenergies decrease, and the reduction of energy is a function of thedistance the particles penetrate. On the one hand the layer of targetmaterial should be suffi-ciently thick to fully utilize the energy ofthe bombarding particles, i. e. thick enough to reduce the energy of thebombarding particles to the threshiold ener-gy for the desired nuclearreaction. On the other hand the layer should be made rela- "tively thinso that it may be snore readily =-coo1ed by the cooling water broughtinto contact with the heat-conductive base and so that thespecificactivity produced will be .high. If the layer is made too thick,the inner portion of the target element serves no useful purpose andacts in effect as a diluent to reduce the activity produced per unitweight of material. This point is particularly important where thetarget element is the same element as the radioisotope resulting fromthe nuclear reaction. Where the radioisotope produced is a differentelement from the target element it may usually be separated therefromand concentrated by chemical means.

The target as described above 'is preferably mounted on a conventionalwater-cooled probe. The particular structure of the probe forms no partof the present invention-and any'of various known probes maybe used suchas, for example, those described by Livingston, "Journal of AppliedPhysics, volume 15, Number 2, pages 128 to 147. In order to point outmore fully the nature centimeter of copper 'plate.

of the present invention the following specific example is given of apreferred method of making the target of the present invention and of asuitable mounting for the target. In this specific example referencewill be made to the accompanying drawing wherein Figure 1 is a top planview of a target and probe and Figure 2 is a vertical section taken onthe lined-'2 of Fig'ure 1.

Referring to the drawingthe probe comprises a hollow chamber l made of asuitable heat-conductive material such as copper. The interior 'ofchamber [0 is supplied with water by an inlet tube 12 and water iswithdrawn from the chamber through an outlet tube H in such mannerthatthe walls of the chamber ID are continuously cooled.

The target :proper, generally designated I8, is afilxed to the uppersurface of the chamber l0 and comprises a thin copper plate I! which maybe, for example, 1 inch square and 1*; inch thick. The upper surface ofplate #8 'is prov-filed with a 'series of AI-shaped ridges --or notchesll. The sides of the ridges 2-0 form an angle umbwt with the surface ofthe =pl-ate and appmxi 'mately ridges are -used.

The plate 18 :forms a heat conductlve base im- -a layer of targetmaterial -22 which is prierwbl y applied to the base t8 before the basei'ssecur'efl -to the probe chamber [0. Application of the targetmaterial #2 to the base *8 is effected spreading i a relatively thinlayer of thallous tellw rate ('I'ITeOI.) .on the ridged surface ofthe-ba'ae and then heating the base and -teuurate Myer to a temperatureof about 800C. to convent the tellurate into a tellu-rite (TITB'Ga) .ofmaterial used is such as to give a tcihfl'it'e layer of approximately250 'milligrams is-manuals": The tellurit'e to form a coherent vitreousmass that adheres strongly to the copper base. whe'texset as chubfor-med is then afiixed to the top of probe whalinber 1-0 in anysuitable manner such as by 'ing and the probe and associated target1aserted in the cyclotron.

The target layer is .preferably so positioned 03 to form a relativelysmall angle with plan! of the beam, 1. e. the angle =0! incidence of'the beam should desirably be of the order of 80 Furthermore the tar-getshould be lengthwise with respect to beam co m the ridges areapproximately parallel to the m and s. maximum surface cf the target isexposed to the beam.

The target is then exposed to a beam 0'1 955 M. E. V. deuterons havingan intensity '0! d'bout one kilowatt for a period =o'f time sumcient toproduce the desired activity 0f rsdioiodine. has been found that an"aotivityof about 0165 malicurie "per hour of exposure may "be obtained.The activity obtained under similar conditions using a copper-cobalttellurium target somewhat with the characteristics 01 the particularcyclotron used but in "general is of "the order of one tenth to one"half of the activity obtained with the target of the present invention.

It is to be understood that the foregoing description is illustrativeonly and that various changes might be made therein and in the scope ofapplication of the present invention. Thus the target described abovemay be "bonibardeH with other types "of particles such asalpha. pardclesand protons and a variety of nuclear relictions may be carried out. Byan appropriate choice of bombarding particles 'andb'eam energies thetarget of the present invention may-be used in the production ofradioisotopes of elements such as antimony, xenon, mercury, thallium,lead and bismuth. In general the present target may be used inconjunction with known nuclear reactions to produce radioisotopes ofelements having the atomic numbers 51, 52, 53, 54, 80, 81, 82 and 83.

Since many embodiments might be made of the above-described inventionand since many changes might be made in the embodiment described, it isto be understood that the foregoing description is to be interpreted asillustrative only and not in a limiting sense.

I claim:

1. A target for use in the production of radioisotopes of elementsselected from the group having the atomic numbers 51 through 54 and 80through 83 by bombardment with high-velocity sub-atomic particles, saidtarget comprising a heat-conductive base having a relatively thin,adherent layer of a, thallium tellurite thereon.

2. A target for use in the production of radioiodine by bombardment oftellurium with highvelocity sub-atomic particles, said target comprisinga heat-conductive base having a relatively thin, adherent layer ofthallous tellurite thereon.

3. A target for use in the production of radioiodine by bombardment oftellurium with highvelocity deuterons, said target comprising a copperbase having a relatively thin, vitreous layer of thallous telluritethereon.

4. A target for use in the production of radioiodine by bombardment oftellurium with highvelocity deuterons, said target comprising a copperbase having a ridged surface, said ridged surface having thereon avitreous layer of thallous tellurite of such thickness as to reduce theenergy of said deuterons to a value just below the threshold energy forproduction of radioiodine from tellurium.

5. A method of making a target for bombardment with high-velocitysub-atomic particles comprising the steps of coating a heat-conductivebase with a relatively thin layer of thallous tellurate and heating saidbase and tellurate layer to convert said tellurate into a firmlyadherent, vitreous layer of thallous tellurite.

6. A method of making a target for bombardment with high-velocitysub-atomic particles comprising the steps of coating :3. heat-conductivebase with a relatively thin layer of thallous tellurate and heating saidbase and tellurate layer to a temperature of about 800 C. to convertsaid tellurate into a firmly adherent, vitreous layer of thalloustellurite.

7. A method of making a target for production of radioiodine bybombardment with high-velocity sub-atomic particles comprising the stepsof coating the ridged surface of a copper plate with a relatively thinlayer of thallous tellurate and heating said base and tellurate layer toform a firmly adherent, vitreous layer of thallous tellurite, thequantity of said tellurate applied being such as to give a telluritelayer of such a thickness as to reduce the energy of said particles to avalue just below the threshold energy for production of radioiodine.

ALLEN F. REID.

REFERENCES CITED The following references are of record in the OTHERREFERENCES Physical Review, vol. 53, page 1015 (1938); ibid., vol. 56,Pp. 965-71 (1939); ibid., vol. 58, page 177 (1940).

